31.6.1 Media Access Controller

The Transmit Block of the Media Access Controller (MAC) takes data from FIFO, adds preamble, checks and adds padding and frame check sequence (FCS). Both half duplex and full duplex Ethernet modes of operation are supported.

When operating in half duplex mode, the MAC Transmit Block generates data according to the Carrier Sense Multiple Access with Collision Detect (CSMA/CD) protocol. The start of transmission is deferred if Carrier Sense (CRS) is active. If Collision (COL) is detected during transmission, a jam sequence is asserted and the transmission is retried after a random back off. The CRS and COL signals have no effect in full duplex mode.

The Receive Block of the MAC checks for valid preamble, FCS, alignment and length, and presents received frames to the MAC address checking block and FIFO. Software can configure the ETH to receive jumbo frames of up to 10240 Bytes. It can optionally strip CRC (Cyclic Redundancy Check) from the received frame before transferring it to FIFO.

The Address Checker recognizes four specific 48-bit addresses, can recognize four different types of ID values, and contains a 64-bit Hash register for matching multicast and unicast addresses as required. It can recognize the broadcast address all-'1' (0xFFFFFFFFFFFF) and copy all frames. The MAC can also reject all frames that are not VLAN tagged, and can recognize Wake on LAN events.

The MAC Receive Block supports offloading of IP, TCP and UDP checksum calculations (both IPv4 and IPv6 packet types supported), and can automatically discard bad checksum frames.